Edible vegetable oils are generally obtained by processing oil seeds. Crude vegetable oils can be obtained from vegetable seeds by solvent extraction. Hexane is the most commonly used extraction solvent. The crude vegetable oils generally contain neutral triglycerides and a host of natural contaminants including phosphotides, sulphurous compounds, free fatty acids, carbohydrates, peptides, oxidized lipids, traces of lower aldehydes and ketones, glycosides of sterols and terpenes, and diverse types of color bodies or dyestuffs. These contaminants are removed from the crude vegetable oils in the course of refining in order to render the vegetable oils palatable.
The recovery of soybean oil from soybeans is particularly desirable. A technique for recovering soybean oil includes several processing steps. The soybean is dehulled and crude soybean oil is extracted with hexane. The extractant (miscella), which includes hexane and crude soybean oil, is further processing to recover palatable soybean oil. The hexane is evaporated from the miscella and the resulting crude soybean oil is degummed. Degumming, as used in conventional processes, refers to the removal of phosphatides and other gums from the oil by adding water and/or acid thereto and centrifuging. The recovered oil can be further refined with water and alkaline (such as NaOH) and centrifuged to remove the fatty acids and gums. The oil resulting from the alkaline refining step can then be bleached to remove color bodies, hydrogenated to render the oils more stable, and deodorized. The techniques of degumming, alkaline refining, bleaching, hydrogenating, and deodorizing are well known in the art. It should be appreciated that each separation step, and particularly centrifuging, results in loss of oil.
Numerous prior art references describe techniques for obtaining vegetable oils by application of membrane technology. For example, U.S. Pat. No. 4,093,540 to Sen Gupta describes refining crude glyceride oils by contacting a composition of glyceride oils and organic solvent under pressure with a semi-permeable ultrafiltration membrane to separate constituents of different molecular weight into retentate and permeate fractions, and contacting the composition or at least one of the fractions with a metal oxide or metalloid oxide adsorbent in a column containing the adsorbent. Additional references which describe the use of membrane technology for separating phospholipids from crude vegetable oils include: U.S. Pat. No. 4,414,157 to Iwama et al.; U.S. Pat. No. 4,533,501 to Sen Gupta; Raman et al., "Membrane Technology", Oils & Fats International, Vol. 10, No. 6, 1994, pages. 28-40; Ziegelitz, "Lecithin Processing Possibilities", Inform, Vol. 6, No. 11, Nov. 1995, pages. 1224-1213; Ondrey et al., "The Skinny On Oils & Fats", Chemical Engineering, Oct. 1997, pages. 34-39; Pioch et al., "Towards An Efficient Membrane Based Vegetable Oils Refining", Industrial Crops & Products, 7 (1998) pages 83-89; Koseoglu et al., "Membrane Applications & Research In The Edible Oil Industry: And Assessment, JAOCS, Vol. 67, No. 4 (April 1990), pages 239-249.